### Symblic matrices

Hi,

is it possible to define totally symbolic matrices.
Something like

```
N,M = var('N,M')
a = matrix(SR, N, M)
b = matrix(SR, M, N)
c = a.dot(b)
```

So that c[i,j] = sum(a[i,k]*b[k,i],k,1,M) or do you really always need an non-symblic matrix size?

If you really always need a non-symblic matrix type, is it possible to define the symbolic variables automatically? Something like

```
a = matrix(SR, 2, 3)
print a[i,j]
output:
a_(i,j)
```

Thanks!

Manuel